Hi,
Both of your SODIMM RAM modules should be the same type and speed i.e. IGb - SODIMM Pc2-4200 ddr2 (ddr2 533 MHz).
Do both 1Gb modules work when installed alone? If not one might be faulty.
Do both 1Gb modules work when installed alone in either memory slot? If not, a memory slot may be faulty. (if a slot hasn't been used before, try gently installing and uninstalling the ram module('''not with the laptop on'''). It may clean the contacts in the slot.
Here is a link to the Acer Aspire Quick Specs Sheet.
http://www.manualslib.com/manual/205264/Acer-Aspire-5315.html#manual